1. Technical Field
The present invention is related to polycyclic polymers and methods for their use as photoresists in the manufacture of integrated circuits. More specifically, the invention is directed to photoresist compositions comprising a polycyclic polymer and a cationic photoinitiator. The polycyclic polymer contains recurring acid labile groups that are pendant from the polymer backbone. The acid labile groups can be selectively cleaved to form recurring polar groups along the backbone of the polymer. The polymers are transparent to short wave lengths of imaging radiation and exhibit resistance to reactive ion etching.
2. Background
Integrated circuits (IC's) are paramount in the manufacture of an array of electronic devices. They are fabricated from the sequential formation of alternating and interconnecting bands of conductive, semiconductive and nonconductive layers on an appropriate substrate (e.g., silicon wafer) that are selectively patterned to form circuits and interconnections to produce specific electrical functions. The patterning of IC's is carried out according to various lithography techniques known in the art. Photolithography employing ultraviolet (UV) light and increasingly deep UV light or other radiation is a fundamental and important technology utilized in the production of IC devices. A photosensitive polymer film (photoresist) is applied over the wafer surface and dried. A photomask containing the desired patterning information is then placed in close proximity to the photoresist film. The photoresist is irradiated through the overlying photomask by one of several types of imaging radiation including UV light, e-beam electrons, x-rays, or ion beam. Upon exposure to radiation, the photoresist undergoes a chemical change with concomitant changes in solubility. After irradiation, the wafer is soaked in a solution that develops (i.e., selectively removes either the exposed or unexposed regions) the patterned images in the photosensitive polymer film. Depending on the type of polymer used, or the polarity of the developing solvent, either the exposed or nonexposed areas of film are removed in the developing process to expose the underlying substrate, after which the patterned exposed or unwanted substrate material is removed or changed by an etching process leaving the desired pattern in a functional layer of the wafer. Etching is accomplished by plasma etching, sputter etching, and reactive ion etching (RIE). The remaining photoresist material functions as a protective barrier against the etching process. Removal of the remaining photoresist material gives the patterned circuit.
In the manufacture of patterned IC devices, the processes of etching different layers on the wafer are among the most crucial steps involved. One method is to immerse the substrate and patterned resist in a chemical bath which attacks the exposed substrate surfaces while leaving the resist itself intact. This "wet" chemical process suffers from the difficulty of achieving well defined edges on the etched surfaces. This is due to chemical undercutting of the resist material and the formation of an isotropic image. In other words, conventional chemical processes do not provide the selectivity of direction (anisotropy) considered necessary to achieve optimum dimensional specifications consistent with current processing requirements. In addition, the wet processes suffer because of the undesirable environmental and safety ramifications.
Various "dry" processes have been developed to overcome the drawbacks of the wet chemical process. Such dry processes generally involve passing a gas through a chamber and ionizing the gas by applying a potential across two electrodes in the presence of the gas. The plasma containing the ionic species generated by the potential is used to etch a substrate placed in the chamber. The ionic species generated in the plasma are directed to the exposed substrate where they interact with the surface material forming volatile products that are removed from the surface. Typical examples of dry etching are plasma etching, sputter etching and reactive ion etching.
Reactive ion etching provides well defined vertical sidewall profiles in the substrate as well as substrate to substrate etching uniformity. Because of these advantages, the reactive ion etching technique has become the standard in IC manufacture.
Two types of photoresists are used in the industry, negative and positive photoresists. Negative resists, upon exposure to imaging radiation, polymerize, crosslink, or change solubility characteristics such that the exposed regions are insoluble to the developer. Unexposed areas remain soluble and are washed away. Positive resists function in the opposite way, becoming soluble in the developer solution after exposure to imaging radiation.
One type of positive photoresist material is based upon phenol-formaldehyde novolac polymers. A particular example is the commercially utilized Shipley AZ1350 material which comprises an m-cresol formaldehyde novolak polymer composition and a diazoketone (2-diazo-1-napthol-5-sulphonic acid ester). When exposed to imaging radiation, the diazoketone is converted to a carboxylic acid, which in turn converts the phenolic polymer to one that is readily soluble in weak aqueous base developing agent.
U.S. Pat. No. 4,491,628 to Ito et al. discloses positive and negative photoresist compositions with acid generating photoinitiators and polymers with acid labile pendant groups. Because each acid generated causes deprotection of multiple acid labile groups this approach is known as chemical amplification which serves to increase the quantum yield of the overall photochemical process. The disclosed polymers include vinylic polymers such as polystyrenes, polyvinylbenzoates, and polyacrylates that are substituted with recurrent pendant groups that undergo acidolysis to produce products that differ in solubility than their precursors. The preferred acid labile pendant groups include t-butyl esters of carboxylic acids and t-butyl carbonates of phenols. The photoresist can be made positive or negative depending on the nature of the developing solution employed.
Trends in the electronics industry continually require IC's that are faster and consume less power. To meet this specification the IC must be made smaller. Conducting pathways (i.e., lines) must be made thinner and placed closer together. The significant reduction in the size of the transistors and the lines produced yields a concomitant increase in the efficiency of the IC, e.g., greater storage and processing of information on a computer chip. To achieve thinner line widths, higher photoimaging resolution is necessary. Higher resolutions are possible with shorter wave lengths of the exposure source employed to irradiate the photoresist material. However, the prior art photoresists such as the phenol-formaldehyde novolac polymers and the substituted styrenic polymers contain aromatic groups that inherently become increasingly absorptive as the wave length of light falls below about 300 nm, (ACS Symposium Series 537, Polymers for Microelectronics, Resists and Dielectrics, 203rd National Meeting of the American Chemical Society, Apr. 5-10, 1992, p.2-24; Polymers for Electronic and Photonic Applications, Edited by C. P. Wong, Academic Press, p. 67-118). Shorter wave length sources are typically less bright than traditional sources which necessitate a chemical amplification approach using photoacids. The opacity of these aromatic polymers to short wave length light is a drawback in that the photoacids below the polymer surface are not uniformly exposed to the light source and, consequently, the polymer is not developable. To overcome the transparency deficiencies of these polymers, the aromatic content of photoresist polymers must be reduced. If deep UV transparency is desired (i.e., for 248 nm and particularly 193 nm wave length exposure), the polymer should contain a minimum of aromatic character.
U.S. Pat. No.5,372,912 concerns a photoresist composition containing an acrylate based copolymer, a phenolic type binder, and a photosensitive acid generator. The acrylate based copolymer is polymerized from acrylic acid, alkyl acrylate or methacrylate, and a monomer having an acid labile pendant group. While this composition is sufficiently transparent to UV radiation at a wave length of about 240 nm, the use of aromatic type binders limits the use of shorter wave length radiation sources. As is common in the polymer art, the enhancement of one property is usually accomplished at the expense of another. When employing acrylate based polymers, the gain in transparency to shorter wave length UV is achieved at the expense of sacrificing the resist's resistance to the reactive ion etch process.
In many instances, the improvement in transparency to short wave length imaging radiation results in the erosion of the resist material during the subsequent dry etching process. Because photoresist materials are generally organic in nature and substrates utilized in the manufacture of IC's are typically inorganic, the photoresist material has an inherently higher etch rate than the substrate material when employing the RIE technique. This necessitates the need for the photoresist material to be much thicker than the underlying substrate. Otherwise, the photoresist material will erode away before the underlying substrate could be fully etched. It follows that lower etch rate resist materials can be employed in thinner layers over the substrate to be etched. Thinner layers of resist material allow for higher resolution which, ultimately, allows for narrower conductive lines and smaller transistors.
J. V. Crivello et al. (Chemically Amplified Electron-Beam Photoresists, Chem. Mater., 1996, 8, 376-381) describe a polymer blend comprising 20 weight % of a free radically polymerized homopolymer of a norbornene monomer bearing acid labile groups and 80 weight % of a homopolymer of 4-hydroxy-.alpha.-methylstyrene containing acid labile groups for use in electron-beam photoresists. As discussed supra, the increased absorbity (especially in high concentrations) of aromatic groups renders these compositions opaque and unusable for short wave length imaging radiation below 200 nm.
The disclosed compositions are suitable only for electron-beam photoresists and can not be utilized for deep UV imaging (particularly not for 193 nm resists).
Crivello et al. investigated blend compositions because they observed the oxygen plasma etch rate to be unacceptably high for free radically polymerized homopolymers of norbornene monomers bearing acid labile groups.
Accordingly, there is a need for a photoresist composition which is compatible with the general chemical amplification scheme and provides transparency to short wave length imaging radiation while being sufficiently resistant to a reactive ion etching processing environment.